One of the abovementioned “intrinsic yield genes”, AN3 (also known as GIF1 and herein also refer to as SYT—synovial sarcoma translocation polypeptide), was identified in search of GRF (growth regulating factor) interactors (Kim and Kende, 2004) and by analysis of narrow-leaf Arabidopsis mutants (Horiguchi et al., 2005). SYT is a homolog of the human SYT (synovial sarcoma translocation) protein and is encoded by a small gene family in the Arabidopsis genome. SYT is a transcription co-activator whose biological function, despite the implication of its chromosomal translocation in tumorigenesis, is still unclear (Clark et al., 1994; de Bruijn et al., 1996). Using the yeast GAL4 system, SYT was shown to possess transactivation activity (Kim and Kende, 2004). This together with yeast two-hybrid and in vitro binding assays demonstrating interaction of SYT with several GRFs (Kim and Kende, 2004; Horiguchi et al., 2005), suggests a role of SYT as transcription co-activator of GRFs. GRF (growth regulating factor) genes occur in the genomes of all seed plants thus far examined and encode putative transcription factors that play a regulatory role in growth and development of leaves (Kim et al., 2003). In support of a GRF and SYT transcription activator and co-activator complex, grf and SYT mutants display similar phenotypes, and combinations of grf and SYT mutations showed a cooperative effect (Kim and Kende, 2004). The SYT mutant narrow-leaf phenotype is shown to result of a reduction in cell numbers. Moreover, ectopic expression of SYT resulted in transgenic plants with larger leaves consisting of more cells, indicating that SYT controls both cell number and organ size (Horiguchi et al., 2005). Although the function of SYT in plant growth regulation is not known, these results show that SYT fulfills the requirements of an “intrinsic yield gene”.
In our ambition to decipher the molecular network underpinning yield enhancement mechanism a genome-wide protein centred approach was undertaken to study SYT interacting proteins in Arabidopsis thaliana cell suspension cultures. The tandem affinity purification (TAP) technology combined with mass spectrometry (MS) based protein identification resulted in the isolation and identification of SYT interacting proteins that may function in the regulation of plant growth (iSYT). Surprisingly, we isolated several proteins belonging to multiprotein complexes. Moreover, many interactors were previously completely uncharacterized. Reports on few of the SYT interactors show that they are implicated in several developmental processes (Wagner & Meyerowitz, 2002; Meagher et al., 2005; Sarnowski et al., 2005; Hurtado et al., 2006; Kwon et al., 2006) but so far none of the identified (iSYT genes have been associated with stimulation of plant growth. Further surprising no specific combination of iSYT polypeptides useful to enhance yield related traits has previously been described.